Immersion tinning from stannate



United States Patent IMlVIERSION TINNING FROM STANN ATE SOLUTIONS Frederick A. Lowenheim, Plainfield, and Herbert B. Forman, Hillside, N. J., assignors to Metal & Thermit Corporation, New York, N. Y., a corporation of New Jersey No Drawing. Application December 6, 1951, Serial No. 260,347

7 Claims. (Cl. 117-130) This invention relates to the process of tin coating articles of cooper and brass and other copper alloys by simple immersion in a salt solution, and without the use of an electric current.

By this immersion process, very thin coatings are obtained and articles are coated wherever the solution can penetrate. Thus, an important application has been the tinning of the inside of copper tubing wherein a flash coating of tin is sufficient to overcome the green water" problem.

It is also commercially feasible to tin brass pins by immersion, thus preventing green stain on textiles. Similarly, many small electrical parts are tinned or color coding and for uniformity in appearance.

Since very small amounts of tin are used by our immersion process, the process becomes important in times of national emergency, when the quantity of tin available is curtailed.

We have discovered that copper tinning may be accomplished by immersing the object to be tinned in an aqueous solution comprising an alkaline stannate and an alkaline cyanide.

In the prior art, the closest procedures of which we are aware are disclosed in (1) U. S. Patent No. 2,159,510 to Pavlish and Sullivan, which employs an inherently unstable alkaline stannite solution; and (2) Bradley U. S. Patent No. 2,282,511 and Sullivan and Pavlish U. S. Patent No. 2,369,620, both of which use stannous salts and thiourea. The Bradley patent referred to above states that stannic salts may be employed but if used, the solution should be of such nature that reduction of the stannic ion to the stannous ion is favored.

The Sullivan and Pavlish cyanide process specified stannous tin and the patentees have stated in Metals and Alloys, May 1940, p. 131 if:

Stannous tin is slowly oxidized in stannite-cyanide solutions to the stannic condition, and in this form it will not coat, but fairly high concentrations of stannic tin can be tolerated with no ill effect, provided the bath contains stannous tin Consequently, immersion tinning revealed in the prior art was considered inoperative when employing stannic salts.

Our process comprises immersing the copper-containing article in a bath of alkaline stannate solution, alkaline cyanide and an excess of alkaline hydroxide. In carrying out our process we determined that there were no stannous salts in our plating solution and also conducted a series of investigations to determine the optimum composition, tem- 2,735,788 Patented Feb. 21, 1956 2 perature and period of immersion for tin'ning under commercial or operating conditions.

Several observations may be made as to the factors determining the effect of the variables involved. First, it was observed that the tinning takes place at all reasonable temperatures, although elevated temperatures are desirable. Second, it was observed that increasing the stannate concentration tends to accelerate the tinning reaction. Third, it was observed that the addition of the alkali hydroxide retards the reaction and so far as speed is concerned is actually deleterious. However, the presence of alkali hydroxide is required for stability, as will appear below. Fourth, it was observed that the cyanide content, to obtain maximum results, should be at least grams per liter. Fifth, it was observed that the tinning takes place under practically all conditions and in all compositions although in some cases from a practical point of view, the reaction was negligible and the depletion rapid.

From our results, we concluded that a composition which is in the optimum range for commercial uses contains approximately alkali stannate 6O g./l., such as potassium or sodium stannate, alkali cyanide 120 g./ 1., such as potassium or sodium cyanide, and alkali hydroxide such as sodium or potassium hydroxide 7.5 g./l., to be used at about 60 C. Since this solution contained a high content of cyanide, the excess alkali was included to stabilize the solution against the evolution of hydrogen cyanide even though the tests revealed that the presence of the hydroxide retarded the reaction. Free alkali is also required to prevent the precipitation of tin hydrate by carbon dioxide absorption.

The above discussed tests were conducted employing both sodium and potassium stannates, sodium and potassium hydroxides and sodium and potassium cyanides. The potassium salts produced a much more vigorous reaction and exhibited superior tinning qualities.

The capacity of the solution was tested by treating copper panels and brass pins until evidence of exhaustion of the bath was encountered. In one such test, ml. of the above solution coated 904 square inches of surface which were satisfactorily plated before any deterioraiton in the results was noted.

We also discovered that potassium carbonate could be substituted for the hydroxide but the solution is not so stable and causes the sludging out of the tin by carbon dioxide absorption.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not to be so limited thereto but is to be construed broadly and restricted solely by the scope of the appended claims.

We claim:

1. The process of forming an immersion coating of tin on the surface of materials selected from the class consisting of copper and copper base alloys which comprises contacting the surface of said materials with an aqueous alkaline solution containing dissolved tin in the stannate condition and a cyanide.

2. The process as set forth in claim 1 in which the solution contains potassium stannate and potassiun cyanide.

3. The process as set forth in claim 1 in which the solution contains sodium stannate and sodium cyanide.

4. The process as set forth in claim 1 in which the solution contains an alkali metal cyanide, an alkaline stannate and an alkali metal hydroxide.

5. The process of forming an immersion coating of tin on the surface of materials selected from the class consisting of copper and copper base alloys which comprises contacting the surface of said materials with an aqueous alkaline solution containing an alkali metal stannate and an alkali metal cyanide, said alkali metal being selected from the class consisting of potassium and sodium.

6. In a process as set forth in claim 5, said solution comprising substantially 60 g./l. of an alkali metal stannatc, 120 g./l. of an alkali metal cyanide and 7.5 g./l. of an alkali metal hydroxide.

7. In a process as set forth in claim 5, said solution being at approximately 60 C.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Wilson et aL: Electroplating 4, pp. 2746 (1951). Geiman: Metal Finishing, vol. 47, issue 9, pp. 52-56 (1949). 

1. THE PROCESS OF FORMING AN IMMERSION COATING OF TIN ON THE SURFACE OF MATERIALS SELECTED FROM THE CLASS CONSISTING OF COPPER AND COPPER BASE ALLOYS WHICH COMPRISES CONTACTING THE SURFACE OF SAID MATERIALS WITH AN AQUEOUS ALKALINE SOLUTION CONTAINING DISSOLVED TIN IN THE STANNATE CONDITION AND A CYANIDE. 